1. Field of the Invention
The present invention relates-to an image forming method for producing a recording liquid having a predetermined density and/or a predetermined color by changing a mixture proportion of an image non-forming liquid with one type or a plurality of types of image forming liquid based on an image signal and leading this recording liquid to an image receiving medium to form an image. The present invention further relates to an image forming apparatus by this method.
2. Description of the Prior Art
There has been proposed an image forming method, in which different colors of ink are mixed based on an image signal in advance and transferred to an image receiving medium such as a print paper (for example, U.S. Pat. Nos. 4,109,282 and 4,614,953, Unexamined Japanese Patent Publication (KOKAI) Nos. 201024/1993, 125259/1995, 207664/1991 and 156131/1997). Such method of using a fluid whose density/color is preset in response to the image signal to form an image is called a pre-mix mode. According to the mode, the density or the color can be adjusted for each pixel based on the image signal, an image quality can be enhanced.
U.S. Pat. No. 4,109,282 discloses a printer having a structure such that a valve called a flap valve is provided in a flow channel for leading two types of liquid, i.e., clear ink and black ink onto a substrate for forming an image. The flow channel for each ink is opened/closed by displacing this valve so that the two types of liquid are mixed in a desired density to be transferred onto the substrate. This enables printout of an image having the same gray scale information as that of image information displayed on a TV screen.
In this reference it is disclosed that a voltage is applied between the flap valve and an electrode provided on a surface opposite to the flap valve and the valve itself is mechanically deformed by an electrostatic attracting force to cause displacement of the valve. The ink is absorbed by a capillary phenomenon between fibers of the print paper. However, since this apparatus joins and mixes the two types of liquid on the substrate immediately after ejection, the system is exactly different from the aforementioned pre-mix mode.
U.S. Pat. No. 4,614,953 discloses an ink jet printer head apparatus in which only desired amounts of a plurality of types of ink having different colors and solvent are led to a third chamber to be mixed therein and flied as an ink droplet. In this reference, a chamber and a diaphragm-type piezoelectric effect device attached to this chamber are used as means for check-weighing a desired amount of ink and a pressure pulse obtained by driving this piezoelectric device is used.
Unexamined Japanese Patent Publication (KOKAI) No. 201024/1993 discloses an ink jet print head including: a liquid chamber filled with a carrier liquid; ink jet driving means disposed in the liquid chamber; a nozzle communicating with the liquid chamber; and a mixing portion for mixing ink to the carrier liquid in this nozzle. Here it is also disclosed that adjusting means for adjusting a mixture amount of ink to obtain a desired value is provided.
Similarly, Unexamined-Japanese Patent Publication (KOKAI) No. 125259/1995 discloses an ink jet recording head including: first and second supplying means for supplying ink having first and second densities, respectively; and controlling means which controls a supply amount of the second ink by the second supplying means so that a desired ink density can be obtained.
In this reference, employment of a micro-pump which has an exclusive heating device and is driven by its heat energy is disclosed as the controlling means. As this micro-pump, an example is disclosed in which the heat energy is generated by the heating-device and a pressure obtained by nucleate boiling caused due to the heat energy is used to drive, for example, a piston-type valve or a cantilever-like valve. Further, this reference describes that an inflow of ink can effectively be controlled in an area where the inflow is particularly small by adopting an actuator consisting of shape memory alloy with this valve.
Unexamined Japanese Patent Publication (KOKAI) No. 207664/1991 discloses an ink jet printer having a structure similar to that disclosed in the above mentioned U.S. Pat. No. 4,614,953, but a third chamber for mixing a plurality of types of ink is not used.
Unexamined Japanese Patent Publication (KOKAI) No. 156131/1997 discloses an ink jet printer comprising a plurality of printer heads for forming an image having multiple colors based on image data. Ink and diluent are mixed at a predetermined mixing ratio to obtain a diluted ink which is jetted from a nozzle so that a recording image is formed on a recording medium. The ink jet printer ejects the diluent from at least one printer head out of a plurality of printer heads when all-white image data, that is, data representing that the mixture amount of ink is too small to realize a clear printing density, is inputted to the plurality of printer heads. As a result, a rapid change in tone (a tone jump) is prevented and the additional consumption of the diluent is suppressed to improve drying characteristics.
The U.S. Pat. No. 4,109,282 discloses a technique of controlling the supply amount of one type of ink to be mixed, that is, black ink. Therefore, a liquid flow rate of the mixed ink having the desired density, that is, a volumetric flow rate per unit time varies in accordance with a change in density.
As a mode in place of an ink jet mode, the applicant and his collaborators have been examining a mode for continuously transferring the ink liquid to an image receiving medium as a continuous flow without making a liquid droplet (hereinafter referred to as a continuous coating mode). Even in this mode, the aforementioned change of the supply amount of the mixed ink results in a disordered and unstable liquid flow. It has been revealed that a fluctuation of a volumetric flow rate (also referred to simply as the flow rate, hereinafter) per unit time of the mixed ink liquid due to the change of the mixing ratio of the inks results in various problems such as remarkable deterioration of quality of a finally formed image. Also when the color is changed by mixing a plurality of types of ink, similar problems occur.
Specifically, it is desirable that the ink liquid is transferred to the image receiving medium as a steady laminar flow. If a disorder or a whirlpool occurs in this flow, the image quality is deteriorated. Moreover, a fluctuation of the supply amount of the liquid leads to formation of coating film having various thickness on the image receiving medium, but it is very difficult to steadily form the coating film having varied or fluctuated thickness depending on a structure of a liquid ejection port. Even if the formation of such coating film is possible, irregularity is generated on the surface of the image, and the transferred liquid having the irregular surface in a liquid state tends to spread and smoothen the irregular surface, which deteriorates the image quality.
Moreover, in the continuous coating mode an image recording liquid needs to be continuously supplied between the recording head and the image receiving medium. However, the recording liquid flows slow on a portion in contact with a surface of the recording head, due to viscosity of the recording liquid. This causes a problem that delay is generated in a recording liquid coating position and the image quality is deteriorated.
In order to avoid the disadvantage, it is proposed to interpose liquids which form substantially no image after image formation, such as a clear liquid (hereinafter referred to as an image non-forming liquid, or simply a clear ink) so that the recording liquid fails to directly contact the surface of the recording head. For this purpose, it is proposed that the recording head is provided with a clear liquid ejection port separately from a recording liquid ejection port, and that the clear liquid is supplied to the portion between the recording head surface and the recording liquid. However, when the ejection portion for the clear liquid is disposed, a recording head structure is complicated and enlarged, and reliability is disadvantageously deteriorated.
Moreover, in the conventional pre-mix mode, even when a plurality of types of ink are mixed in advance at a mixture proportion based on the image signal, an ink velocity distribution in an ink flow path or channel inevitably changes in a position close to an inner wall of the ink channel and a far position until the ink reaches the ink ejection port. Specifically, an ink phase is advanced in the vicinity of a center of the ink channel, and is delayed in the vicinity of a peripheral wall. Therefore, the ink for one pixel is ejected with a time deviation in a portion close to the ink channel inner wall and in a portion far therefrom.
FIG. 22 is a diagram schematically showing image quality deterioration by such phenomenon. In FIG. 22, the image receiving medium (print paper) moves downward (in a direction of an arrowhead), and a plurality of ink ejection ports (four ports in this example) are arranged in a horizontal direction (direction crossing at right angles to the arrowhead).
In this case, with movement of the image receiving medium in the arrowhead direction, the ink for each pixel is adjusted beforehand to provide a predetermined density (and/or color), but with the ink flowing in the ink channel, a flow velocity in the portion in contact with the inner wall of the ink channel becomes slow, and in the portion apart from the inner wall the flow velocity becomes fast. The ink has viscosity, and a flow velocity distribution therefore forms a substantial rotation parabolic surface when the ink channel has a circular section.
Therefore, a state of the ink ejected for a certain pixel can be divided to the following three stages. First, in a first stage (shown by A in FIG. 22), a part of image forming ink (colored ink) is carried by a fast flow in the vicinity of the center of the ink channel, while diffused in an image non-forming ink (clear ink) to be thinned. In a second stage (shown by B), a normal amount of image forming ink enters, and a correct density (and/or color) is obtained. In a third stage (similarly shown by C) after supply of the image forming ink stops, the ink in the vicinity of the inner wall of the ink channel is delayed and ejected.
As a result, one pixel density (color) is blurred forward and backward in parallel with a moving direction of the image receiving medium, and the image quality is deteriorated. Moreover, it can also be said that the delay until image forming ink reaches the ink ejection port changes with a position in a diametric direction of the ink channel, and therefore a rise and fall of density (color) change at the ink ejection port fail to become sharp.
The present invention has been accomplished in consideration of the aforementioned circumstances, and an object thereof is to provide an image forming method for producing a recording liquid having a desired density and/or color by mixing an image non-forming liquid with one type or a plurality of types of image forming liquid having different densities and/or colors and transferring the recording liquid to an image receiving medium to form an image thereon.
Another object of the present invention is to provide an image forming method which prevents the recording liquid from being disordered before reaching the image receiving medium and which can prevent deterioration of an image quality.
Further object of the present invention is to provide an image forming method in which the recording liquid is transferred to the image receiving medium without being disordered by a recording head simple in structure and high in reliability, and the image quality can be enhanced.
Still another object of the present invention is to provide an image forming method which prevents occurrence of blur of pixel density (or color) in a pre-mix mode and which can enhance the image quality.
Moreover, an object of the present invention is to provide an image forming apparatus for producing a recording liquid having a desired density and/or color by mixing an image non-forming liquid with an image forming liquid and transferring the recording liquid to an image receiving medium to form an image thereon.
Another object of the present invention is to provide an image forming apparatus which prevents a laminar flow structure of the image non-forming liquid and image forming liquid in the recording liquid from being disordered before reaching the image receiving medium and which can prevent deterioration of the image quality.
Further object of the present invention is to provide an image forming apparatus in which the recording liquid is transferred to the image receiving medium without being disordered by a recording head simple in structure and high in reliability, and the image quality can be enhanced.
Still another object of the present invention is to provide an image forming method which prevents occurrence of blur of pixel density (or color) in the pre-mix mode and which can enhance the image quality.
In one aspect of the present invention, there is provided an image forming method for forming an image on an image receiving medium with a recording liquid which includes an image forming liquid for finally forming an image and an image non-forming liquid for forming no image after image formation, a mixing ratio of the image forming liquid and the image non-forming liquid being varied based on an image signal, said method comprising steps of:
a) allowing said image non-forming liquid to flow in a recording liquid channel to an ejection port;
b) supplying said image forming liquid into said image non-forming liquid flowing in said recording liquid channel to form said recording liquid so that the image forming liquid in the recording liquid is prevented from contacting an inner wall surface of said recording liquid channel;
c) delivering said recording liquid to the ejection port; and
d) ejecting said recording liquid from the ejection port to the image receiving medium to form the image thereon.
In the image forming method of the present invention, the image forming liquid is ejected or extruded into the flow of the image non-forming liquid so that the image forming liquid fails to contact the inner wall of the recording liquid channel. The image forming liquid can be transferred to the image receiving medium in a wrapped or enveloped state in the image non-forming liquid, the image forming liquid is prevented from being disordered before reaching the image receiving medium, and the image quality can be prevented from being deteriorated.
The image non-forming liquid is a liquid by which substantially no image is formed after the image formation (i.e., after application of the recording liquid). Examples of the liquid include a liquid which substantially becomes transparent after dried out, an ultraviolet-curing liquid which substantially becomes transparent after cured, a liquid which is cured by thermal or chemical reaction and substantially becomes transparent, a liquid which evaporates after the image formation and substantially disappears, and the like. Here, this image non-forming liquid will also be referred to simply as the clear liquid or clear ink.
The image non-forming liquid flows as a laminar flow in a recording liquid channel, while the image forming liquid is extruded into the laminar flow entirely as it is as possible, and the image forming liquid enclosed in the clear liquid may be led to a recording liquid ejection port. The image forming liquid can be extruded in a direction substantially crossing at right angles to the flow of the clear liquid, or can be extruded with inclination toward a flow direction or in parallel with a flow direction.
The image forming liquid may be of one color, or the plural image forming liquid having different colors may be extruded into a common clear liquid (image non-forming liquid) so that the color as well as the density can be varied. The image forming liquid is preferably extruded to reach the vicinity of a center of the recording liquid channel, but when the liquid is extruded to a position apart from the inner wall of the recording liquid channel by at least 4 xcexcm, the image forming liquid enveloped with in the clear liquid smoothly moves without contacting the inner wall. A continuous coating mode is preferable in which for the recording liquid the image forming liquid enveloped in the clear liquid is continuously transferred to the image receiving medium. However, this method can also be applied to an ink jet mode. in which the recording liquid (image forming liquid wrapped in the image non-forming liquid) is jetted flied as a liquid droplet and transferred to the image receiving medium.
In a preferred embodiment, a viscosity of the image non-forming liquid is set to be smaller than that of the image forming liquid. Thereby, the recording liquid flowing in the recording liquid channel is led to the recording liquid ejection port as the laminar flow such that the image forming liquid fails to contact the inner wall of the recording liquid channel.
In order to obtain the flow in the recording liquid channel as the laminar flow, Reynolds number in this flow channel system may be set to 100 or less. When the recording liquid channel is linear, the recording liquid easily flows straight as a stable laminar flow, and the image forming liquid (colored ink) may be added midway. To lower the viscosity of the image non-forming liquid, a substance having low viscosity may of course be selected, but there is also a method of mixing a fine bubble in this ink (image non-forming liquid) to lower an apparent viscosity.
The inventors have known as an experiment result that when the value P defined by the following equation results in 81 mmxe2x88x921 or more, the image quality considerably becomes satisfactory, and when the value P results in 195 mmxe2x88x921 or more, the image quality further becomes satisfactory:             n      +              (                              h            2                    -          1                )                    n      ⁢              xe2x80x83            ⁢      L        =  P
where, h=R/Ri; n=xcexcc/xcexci; R is an apparent diameter of recording liquid channel (unit: m); Ri is an apparent flow diameter of the image forming liquid (unit: m); and xcexcc and xcexci is fluid viscosities of the image non-forming liquid and the image forming liquid, respectively (unit: pascalxc2x7second (Paxc2x7sec)). Moreover, L denotes a distance (unit: mm) from a confluent position of the image forming ink until the ink is detached from an ink ejection port.
In another aspect of the present invention, there is provided an image forming apparatus for forming an image on an image receiving medium with a recording liquid which includes an image forming liquid for finally forming an image and an image non-forming liquid for forming no image after image formation, a mixing ratio of the image forming liquid and the image non-forming liquid being varied based on an image signal, said apparatus comprising:
an ejection port for ejecting said recording liquid onto the image receiving medium;
a recording liquid channel for delivering the image non-forming liquid to said ejection port;
an extruding port for extruding the image forming liquid into the image non-forming liquid to form said recording liquid, said extruding port being disposed apart from an inner wall of the recording liquid channel through which said image non-forming liquid flows and opened in a flow of the image non-forming liquid; and
an image forming liquid channel for supplying the image forming liquid to said extruding port.
According to the present image forming apparatus, the image forming liquid enveloped in the image non-forming liquid is transferred to the image receiving medium. An image non-forming liquid ejection port for preventing the image forming liquid from directly contacting the recording head does not have to be separately disposed on the recording head, and the structure of the recording head can be simplified. Therefore, the recording head can be miniaturized and the reliability can be enhanced.
The image forming liquid extruding port may be opened on a tip end of a projection disposed on the inner wall of the recording liquid channel. In such construction, the extruding port may be opened in a vertical direction or obliquely toward a downstream direction with respect to the flow direction of the image non-forming liquid. This projection may be formed in a cylindrical shape, an elliptical shape, a vessel or ship bottom shape, and other various shapes, but is preferably formed so that the flow of the image non-forming liquid fails to be disordered.
The image forming liquid may be extruded or introduced parallel to the flow direction of the image non-forming liquid. For example, the image non-forming liquid is ejected from an end surface of one end opposite to the ejection port of the recording liquid channel, and the image forming liquid is extruded from the vicinity of the center of the end surface to be joined with the image non-forming liquid. The color and density of the recording liquid may be adjusted by adding a plurality of image forming liquids into the common image non-forming liquid and changing the mixture proportion thereof.
In further aspect of the present invention, there is provided an image forming apparatus for forming an image on an image receiving medium with a recording liquid which includes an image forming liquid for finally forming an image and an image non-forming liquid for forming no image after image formation, a mixing ratio of the image forming liquid and the image non-forming liquid being varied based on an image signal, a viscosity of the image non-forming liquid being smaller than the viscosity of the image forming liquid, said apparatus comprising;
a recording liquid channel for delivering the recording liquid to said ejection port, the image non-forming liquid and the image forming liquid being joined in the recording liquid channel to form the recording liquid as a laminar flow to said ejection port;
means for supplying the image non-forming liquid to said recording liquid channel on an upstream side from a confluent position of the image non-forming liquid and the image forming liquid;
an image forming liquid extruding port disposed apart from a wall surface of said recording liquid channel and opened in the image non-forming liquid; and
means for supplying the image forming liquid to the image forming liquid extruding port.
By separately providing supply systems of the image forming liquid and image non-forming liquid with flow rate control valves, supply amounts of the respective liquids may be controlled. It is preferable to form the recording liquid channel in a substantially vertically straight line with respect to an opening surface of the recording liquid ejection port, and to dispose the image forming liquid extruding port on a tip end of a projection protruded from the inner wall of the recording liquid channel. With such arrangement, the linear recording liquid channel forms a stable laminar flow, and the recording liquid can smoothly be transferred to the image receiving medium from the recording liquid ejection port.
Distance L, which is a distance between a position where the image forming liquid is joined to the image non-forming liquid and a position where the joined recording liquid is detached from the recording liquid ejection port, is preferably set to be as small as possible. When this distance L is large, a laminar flow state is easily collapsed during movement of the recording liquid along the distance L, and a phase difference between the image forming liquid and image non-forming liquid at the position of the recording liquid ejection port is enlarged by ink viscosity. Moreover, mixture by diffusion between both liquids (inks) also proceeds. This deteriorates the image quality.
The distance L may be determined by considering the flow velocity of the ink (image forming liquid and image non-forming liquid), and other conditions. For example, the ink flow velocity and distance L have to be set with respect to the same image signal in such a manner that the phase difference in the recording liquid ejection port between the image forming liquid and the image non-forming liquid becomes sufficiently small. It is preferable to set the ink flow velocity and distance L in such a manner that the phase difference is 20 xcexcm or less. In this case, since the image forming liquid and image non-forming liquid with a small phase difference with respect to the same image signal can be led to the recording liquid ejection port, the image quality can further securely be prevented from being deteriorated.
In still further aspect of the present invention, there is provided an image forming method for ejecting an image non-forming ink for finally forming no image and an image forming ink for finally forming an image from an ink ejection port by changing a mixture proportion based on an image signal, and transferring the inks to an image receiving medium to form the image thereon, said method comprising steps of:
setting a viscosity of said image non-forming ink to be smaller than a viscosity of said image forming ink;
joining said image forming ink to said image non-forming ink midway in an ink channel for feeding said image non-forming ink as a laminar flow to the ink ejection port so that said image forming ink is prevented from contacting an inner wall of said ink channel; and
delivering the inks as the laminar flow to the ink ejection port.
In the present invention, the image formed on the image receiving medium includes graphical intelligence patterns such as alphanumeric characters, graphical display, line art, circuit pattern and other image information.